The present invention relates to the field of cleaning hydraulic circuits and more particularly the hydraulic circuit serving to control a hydraulic element such as, for example, a cylinder/damper assembly (known as a pitch trimmer) used for controlling the angle of a landing gear assembly and providing shock absorption on landing.
Modern aircraft have several landing gear assemblies. An aircraft such as the A380 marketed by Airbus (registered trademarks) has four landing gear assemblies, two at the level of the cockpit and another two at the level of the wings. Each of these landing gear assemblies has a pitch trimmer used to control the angle of the landing gear assembly for its retraction into the landing gear housing and to provide shock absorption on landing.
FIG. 1 shows diagrammatically the structure of the hydraulic control circuit. A hydraulic element 1.2, typically a pitch trimmer, is connected to a hydraulic control circuit 1.1, typically on board an aircraft. The hydraulic control circuit makes it possible to control the hydraulic element. A first circuit 1.3, known as the high-pressure circuit, makes it possible to transmit a force, by means of a hydraulic fluid, to the element 1.2. A second circuit 1.4, known as the low-pressure circuit, allows the hydraulic fluid to be returned to the control circuit 1.1.
When the hydraulic circuit is first filled, fluid is sent from the control circuit and progressively fills the entire the circuit. It proceeds throughout the circuit, finally reaching the controlled hydraulic element, in this case the pitch trimmer 1.2. When this takes place, particularly when the hydraulic fluid reaches the elements of the circuit for the first time, impurities may be collected by the fluid. Said impurities tend to accumulate in certain places in the circuit and particularly at the hydraulic element situated at the end of the circuit. There is then a risk that these impurities will interfere with the correct operation of the element and may even cause a blockage.
It is also important for a hydraulic control circuit to be completely free of air. Since air is compressible, the presence of an air bubble within the hydraulic circuit interferes with the effective transmission of force by the fluid between the control circuit and the controlled hydraulic element. Moreover, in the case of the pitch trimmers used in aviation, the tubing used to convey the fluid to the controlled element is typically made from Teflon-lined Kevlar. FIG. 2 shows the typical structure of a tube used in the hydraulic circuit in aviation. The tube 2.3 is connected to the controlled element 2.1 by a hydraulic coupling 2.2. The tube 2.3 is constituted by a Kevlar sleeve 2.6 lined with a thickness of Teflon 2.7. When the circuit is filled, fluid 2.4 comes into contact with air 2.5 in order to replace it. This type of tubing is designed to withstand hydraulic pressure but is very fragile in the presence of air in the circuit. As the controlled element is typically located at the end of the line of the hydraulic circuit, an accumulation of air tends to occur, leading to deterioration of the Teflon, which causes seepage. Inadequately purged tubes are therefore systematically damaged and must be replaced.